Medical examiner office personnel in New York City have launched a massive review of more than 800 rape cases to find out if evidence was overlooked or mishandled by a lab tech.

So far supervisors have discovered 26 problem cases, mostly involving instances in which additional biological evidence was detected, the New York Times reports. The Times notes that in seven cases, full DNA profiles were able to be created, including one that matched a convicted offender’s DNA profile and leading to an indictment some 10 years after the evidence was first collected.

Despite the lab’s reputation as being a leader in the forensic technology field, the Times notes that this review of a single technician’s caseload “underscores how DNA evidence, widely perceived as being nearly irrefutable proof of guilt or innocence, can still be subject to human error.”

There are currently no concerns that the lab tech’s mistakes led to false positives. Instead, the investigation is finding false negatives. “We do know that nobody was wrongfully convicted,” Dr. Mechthild Prinz, the office’s director of forensic biology, is quoted saying.

The lab isn’t identifying the lab tech, other than to note the individual worked for the lab for nine years and resigned in 2011 when the office tried to fire her.